The evolution of computer and electronic systems has demanded ever-increasing levels of performance. In most regards, the increased performance has been achieved by electronic components of ever-decreasing physical size. The diminished size itself has been responsible for some level of increased performance because of the reduced lengths of the paths through which the signals must travel between separate components of the systems. Reduced length signal paths allow the electronic components to switch at higher frequencies and reduce the latency of the signal conduction through relatively longer paths.
One technique of reducing the size of the electronic components is to condense or diminish the space between the electronic components. A diminished size also allows more components to be included in a system, which is another technique of achieving increased performance because of the increased number of components.
A particularly effective approach to condensing the size between electronic components is to attach multiple semiconductor integrated circuits or “chips” on printed circuit boards, and then stack multiple printed circuit boards to form a three-dimensional configuration or module. Z-axis interconnectors are extended vertically, in the z-axis dimension, between the vertically stacked printed circuit boards, each of which is oriented in the horizontal x-axis and y-axis dimensions. The interconnectors, in conjunction with conductor traces of each printed circuit board, connect the chips of the module with short signal paths. The relatively high concentration of chips, which are connected by the three-dimensional, relatively short length signal paths, are capable of achieving very high levels of functionality.
The z-axis interconnectors contact and extend through plated through holes or “vias” formed in each of the printed circuit boards. The chips of each printed circuit board are connected to the vias by conductor traces formed on or within each printed circuit board. The vias are formed in each individual printed circuit board of the three-dimensional modules at similar locations, so that when the printed circuit boards are stacked in the three-dimensional module, the vias of all of the printed circuit boards are aligned vertically in the z-axis. The z-axis interconnectors are then inserted vertically through the vertically aligned vias to establish an electrical contact and mechanical connection between the circuit boards, thus assembling the module.
A number of different types of z-axis interconnectors have been proposed. One particularly advantageous type of z-axis interconnector is known as a “twist pin.” An example of a prior art twist pin 50 is shown in FIG. 1. The twist pin 50 is formed from a length of wire 52 which has been formed conventionally by helically coiling a number of outer strands 54 around a center core strand 56 in a planetary manner, as shown in FIG. 2. At selected positions along the length of the wire 52, a bulge 58 is formed by untwisting the outer strands 54 in a reverse or anti-helical direction. As a result of untwisting the strands 54 in the anti-helical direction, the space consumed by the outer strands 54 increases, causing the outer strands 54 to bend or expand outward from the center strand 56 and create a larger diameter for the bulge 58 than the diameter of the regular stranded wire 52. The laterally outward extent of the bulge 58 is illustrated in FIG. 3, compared to FIG. 2. The strands 54 and 56 of the wire 52 have the necessary mechanical characteristics to maintain the shape of the wire in the stranded configuration and to allow the outer strands 54 to bend outward at each bulge 58 when untwisted.
The bulges 58 are formed at selected predetermined distances along the length of the wire 52 to contact vias 60 in printed circuit boards 62 of a three-dimensional module 64, as shown in FIG. 4. Contact of the bulge 58 with the vias 60 is established by pulling the twist pin 50 through an aligned vertical column of vias 60. The outer strands 54 of the wire 52 have sufficient resiliency characteristics so that the outward protruding bulge 58 resiliently presses against an inner surface of a sidewall 66 of each via 60, thereby establishing the electrical and mechanical connection between the twist pin 50 and the via 60, as shown in FIG. 5.
To insert the twist pins 50, a leader 68 is extended through the vertically aligned vias 60 of the vertically stacked printed circuit boards 62 (see FIG. 8). The strands 54 and 56 at a terminal end 70 of the leader 68 have been welded or fused together to form a rounded end configuration 70 to facilitate insertion of the twist pin 50 through the column of vertically aligned vias. The leader 68 is of sufficient length to extend through all of the vertically aligned vias 60 of the assembled stacked printed circuit boards 62, before the bulge 58 which adjoins the leader 68 makes contact with the uppermost via 60 of the outermost printed circuit board 62. The terminal end 70 of the leader 68 extends below the lowermost one of the vertically aligned vias of the lowermost printed circuit board 62.
The terminal end 70 of the leader 68 is gripped from below and is pulled downwardly, causing the bulges 58 to move downwardly through the vertically aligned vias 60 until the bulges 58 are all aligned and in contact with the vias 60 of the stacked printed circuit boards. To position the bulges in contact with the vertically aligned vias, the lower leading bulges 58 closest to the leader 68 are pulled into and out of the vertically aligned vias until the twist pin 50 arrives at its final desired and assembled position. The resiliency of the bulges 58 allows them to move in and out of the vias 60 without losing their ability to make firm contact with the sidewall of the via in the final assembled position. Once the twist pin is in the final assembled position, the leader 68 is cut off flush or sub-flush at a predetermined length which is slightly beyond the lower surface of the lower printed circuit board of the module 64, for example no greater than 0.015 inch beyond the lower surface.
A tail 72 at the other end of the twist pin 50 extends a short distance above the upper trailing bulge 58. The strands 54 and 56 at an end 74 of the tail 72 are also fused together. The length of the tail 72 positions the other end 74 of the twist pin 50 at a similar position above the upper circuit board compared to the position where the leader 68 was cut off relative to the lower circuit board. Allowing the tail 72 and the remaining portion of the leader 68 to extend slightly beyond the outer printed circuit boards 62 of the module 64 facilitates gripping the twist pin 50 when removing it from the module 64 to repair or replace any defective components.
The twist pins are typically of a very small size. The most common sizes of strands 54 and 56 of the helically-coiled wire 52 used is to form twist pins 50 are about 0.0016, 0.0033 and 0.0050 in. in diameter. The diameters of the coiled strands of the wire 52 formed from the these sizes of strands 54 and 56 are 0.005, 0.0010, and 0.0015 in., respectively. The typical length of a twist pin having four to six bulges which extends through four to six printed circuit boards will be about 1 to 1.5 inches, with the leader constituting about half of this length. The outer diameter of each bulge 58 will be approximately two to three times the diameter of the stranded wire 52 in the intervals 76. The tolerance for locating the bulges 58 between intervals 76 is in the neighborhood of 0.002 in. The weight of a typical four-bulge twist pin is about 0.0077 grams, making it so light that handling the twist pin is very difficult. It is not unusual that a complex module formed by 4 in. by 4 in. printed circuit board 62 may require the use of as many as 22,000 twist pins. Thus, the relatively large number of twist pins necessary to assemble each three-dimensional module necessitates an ability to pull each twist pin into the desired position and to cut the leader off in an efficient and rapid manner.
The common technique for pulling and cutting the twist pins involves gripping the protruding end 70 of the leader 68 with one machine and pulling the leader downward. Thereafter, a separate machine moves in from the side and cuts the leader. For a large number of twist pins to be assembled efficiently in a relatively short amount of time, the movements of the separate pulling machine and cutting machine must be coordinated with one another. Coordinating the functionality of two independently operating machines is difficult, and generally requires very complex electronic sensors and controllers. Furthermore, because the operation of the pulling and cutting machines are independent of one another, the functionality of one machine may adversely influence the proper functionality of the other machine. Such prior art pulling and cutting machines are relatively large devices which are intended to be used in a stationary manner. The stacked printed circuit boards must be positioned and oriented relative to the stationary machines. Consequently, it is impossible, expensive or extremely inconvenient to assemble three-dimensional circuit modules other than by use of this type of assembly-line equipment. The expense of programming the separate stationary machines is not conducive to the use of the z-axis interconnectors to assemble relatively smaller numbers of modules.
These and other considerations pertinent to the fabrication of twist pins have given rise to the new and improved aspects of the present invention.